The prior art teaches equipping vehicles with “variable displacement,” “displacement on demand,” or “multiple displacement” internal combustion engines in which one or more cylinders may be selectively “deactivated,” for example, to improve vehicle fuel economy when operating under relatively low-load conditions. Typically, the cylinders are deactivated through use of deactivatable valve train components, such as the deactivating valve lifters as disclosed in U.S. patent publication no. US 2004/0244751 A1, whereby the intake and exhaust valves of each deactivated cylinder remain in their closed positions notwithstanding continued rotation of their driving cams. Combustion gases are thus trapped within each deactivated cylinder, whereupon the deactivated cylinders operate as “air springs” while a corresponding increase in intake manifold pressure operates to reduce engine pumping losses. When vehicle operating conditions are thereafter deemed to require an engine output torque greater than that achievable without the contribution of the deactivated cylinders, as through a heightened torque request from the vehicle operator based upon a detected intake manifold air pressure representing a current engine load, the deactivatable valve train components are returned to their nominal activated state to thereby “reactivate” the deactivated cylinders.
To provide an improved driving experience, the prior art has sought to reduce driver-perceptible engine torque disturbances during mode transitions by adjusting spark timing, throttle position, and fuel before, during, and/or after the transition, until a steady state at the new operating mode has been achieved.
Additionally, if, at the time of a transition from a full-displacement operating mode to a cylinder-deactivation “partial-displacement” operating mode, a vehicle is equipped with a torque converter that is not already operating at a maximum slip rate, the prior art teaches allowing greater torque converter slip during the transition to thereby dampen the torque pulses generated during the transition. After the transition, the slip rate is lessened, for example, to a lockup condition to thereby promote improved vehicle fuel economy in the ensuing partial-displacement mode.
Unfortunately, in some instances, such slip rate reductions after transitioning to a partial-displacement mode undesirably increase vehicle noise, vibration, and harshness (NVH) levels, particularly at lower engine and vehicle speeds, thereby reducing the system's transparency to vehicle passengers.